I'm not a materials (or IC) guy, but my guess is that fracture toughness isn't going to be huge issue. Once the wafer is made, they're all going to be treated pretty gently. The tricky step is actually making the wafer. My understanding is that silicon wafers are normally manufactured by growing a giant silicon crystal ingot, and then basically slicing them into wafers. My understanding of graphene is that since they more or less have to be grown in sheets, then there isn't much of a subtractive process to put high stress on the material.